Production of single crystal material



Dec. 8, .1970 1', J, HURL E-TAL 3,546,027

PRODUCTION OF SINGLE CRYSTAL MATERIAL 2 Sheets-Sheet 1 Filed may a, 1969 FIG.

CONCENTRATION- FIG. 3

Inventors M 04.4 xv Luv/ 153 /ai (aim ttorney;

Dec. 8,1970 v 0.11.1. HURLE ETA!- 3,546,027

PRODUCTION OF SINGLE CRYSTAL MATERIAL Filed May 1969 z Sheets-Sheet 2 [Ill] Growth directlon. l3

Fig. 4.

Inventors Attorney;

US. Cl. 1481.6 4 Claims ABSTRACT OF THE DISCLOSURE A system for growing a metastable phase of a material at the expense of a stable phase of the same material by the use of a thin liquid alloy zone and the use of an externally imposed thermodynamic driving force.

This application is a continuation-in-part of application Ser. No. 442,934, filed Mar. 26, 1965, now US. Pat. 3,460,998; issued on Aug. 12, 1969.

The present invention relates to the production of single crystal material.

It is an object of the invention to provide a new method for the production of single crystal material. The materials concerned belong to a class of materials which undergo polymorphic or order/disorder transformations under certain environmental conditions and for which it is possible to provide a thermodynamic driving force the effect of which is to make the desired phase the stable phase under the environmental conditions. There must exist some solvent which lowers the liquidus temperature and has negligible solubility in the solid phases. For example, by this process grey tin may be grown from white tin, diamond from graphite and epsilon-cobalt from alphacobalt. The expression environmental conditions is here taken to include all the physical environment, for example, temperature, pressure, electric current, and magnetic, electric and gravitational field and includes the thermodynamic driving force which may have the effect of varying any of the environmental conditions.

According to the present invention there is provided, in a system having a metastable phase and a stable phase of the same material separated by a thin liquid alloy zone, the use of an externally imposed thermodynamic driving force so directed as to cause the growth of the metastable phase at the expense of the stable phase.

US. Pat. No. 2,813,048 due to W. G. Pfann, teaches the use of a temperature gradient for bringing about composition or physical changes in a body of material. Such changes may be the distribution of the significant impurities in extrinsic semiconductor material, the purification of semiconductor material or the treatment of magnetic material or organic or inorganic salt solutions. All such processes are dependent upon the composition or the crystalline form of the material concerned.

US. Pat. No. 2,932,562, also due to W. G. Pfann, teaches the heating of a comparatively thick molten zone by Joule heat by passing an electric current through a bar of the material.

US. Pat. No. 3,378,409 due to the inventors in the instant application teaches the passing of an electric current through a piece of material subject to constitutional supercooling in order to cause a thin alloy zone to pass through the material for the purpose of crystallisation. This process is carried out isothermally.

None of the above three US. patents describe any method of growing one phase of the material at the expense of another phase of the same material. In all of United States Patent 3,546,027. Patented Dec. 8, 1970 ice them it is the same phase of the material that is grown albeit with a different crystalline form or a different impurity distribution.

On the other hand, co-pending US. patent application Ser. No. 442,934, now US. Patent 3,460,998 by the inventors in the instant, and of which the instant application is a continuation-in-part, teaches a process for the growth of a stable phase of a material at the expense of a metastable phase by relying upon the lower chemical potential of the stable phase. In other words, the difference in chemical potential between the two phases effects the desired results, which is motion of the thin alloy zone, automatically.

However, the present application provides a method of growing the metastable phase (which has a higher chemical potential) at the expense of the stable phase by imposing an external thermodynamic driving force.

It the metastable phase is labelled M and the stable phase is labelled S and their chemical composition (which is the same in both cases) is labelled X then the liquid alloy zone must be a solution of X dissolved in an appropriate solvent Y. Both X and Y can be either single chemical elements or multicomponent mixtures or compounds. Y must have the property that (i) alloys XY are molten at temperatures below the melting point of M and S at the operating pressure; (ii) Y is insoluble or virtually insoluble in solid M and S.

Growth of a single crystal of S on the single crystal seed of S at the expense of the M charge is achieved by the controlled motion of the liquid alloy zone along the bar away from S, and vice versa in a manner described below.

Embodiments of the invention will be described by way of example and with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic representation of an arrangement for the production of single crystal material;

FIG. 2 is an isobaric surface of the phase diagram of a hypothetical binary alloy XY;

FIG. 3 is a graphical representation of the concentration of Y in the liquid alloy zone plotted across the length of the zone; and

FIG. 4 is a diagrammatic representation of a process embodying the invention.

In FIG. 1 a bar 1 consists of a seed 2 of a material A separated from a charge of 3 of a material B by a sandwich 5 of alloy which is liquid at the temperature and pressure of performance of the process. The materials A and B are different solid polymorphs of the same substance. The sandwich 5 of alloy constitutes a thin alloy zone which is caused to move in the direction shown by an arrow 7, as described below.

Under these conditions, if the bar is maintained at a uniform temperature and pressure such that A is the thermodynamically stable phase and such that some mixture of X and Y, to form the thin alloy zone, is molten, then the equilibrium conditions are as follows.

By definition, A will have a lower chemical potential than B. For a planar solid-liquid interface a gradient of chemical potential of average value (,u p. )/L will exist across the thin liquid alloy zone, Where ,u and are the chemical potentials of the materials A and B respectively and L is the length of the zone measured normal to the solid/liquid interfaces. The gradient of chemical potential will cause the zone to migrate away from the seed crystal. Thus the velocity of the zone will increase with decreasing zone length and in practice the zone length is made sufficiently small for an adequate velocity to be achieved. In general L will be in the range 10- to 10" cm. In other words the process leads to the growth of that polymorph which is the thermodynamically stable one at the operating temperature and pressure.

However by providing externally another thermodynamic driving force which opposes and overrides (lL ,U. )/L it is possible to grow the metastable B phase at the expense of the stable A phase. Suitable external driving forces are an electric current, a temperature gradient or a pressure gradient. It is assumed that the metastable phase B does not spontaneously transform to the stable phase A.

The principles will now be described with reference to FIG. 2, which is an isobaric surface of the phase diagram of a hypotherical binary alloy XY. The substance to be crystallized exists in two polymorphic forms A and B, and the transition temperature between the forms is T The liquid Y dissolves X but is virtually insoluble in solid X. The liquidus curve 9 has a discontinuity in slope at the transition temperature T The slopes of the two portions of the liquidus curve 9 are designated m and i11 We consider an experiment performed in the A stable region of the phase field and to determine the condition of metastable equilibrium between B and the liquid we extrapolate the B liquidus curve into the A phase field at 11.

Consider initially the isothermal Example 1 above, in which the sandwich is at a temperature T. If, initially, the concentration of Y in the liquid alloy zone is C then with respect to the A solid the liquid is supercooled and with respect to the B solid super-heated. The A/liquid interface therefore freezes and the B/liquid interface dissolves until the concentration in the liquid zone reaches C at the A/ liquid interface and C at the B/liquid interface. This is represented diagrammatically in FIG. 3, where concentration is plotted against distance across the zone. Both interfaces are now in local equilibrium. This is not an equilibrium arrangement however because diffusion will take place across the liquid zone in the concentration gradient of average value (C C )/L. A steady transfer of Y across the zone from the A/liquid to the B/liquid interface occurs and zone moves along the bar away from the seed end.

If now a direct electric current is passed axially through the sandwich in that direction which produces a differential migration of Y ions towards the A/liquid interface, then for some magnitude of the current the electrostatic force on the Y ions will exactly balance the diffusion force and the concentration gradient (C -C )/L will be stabilised and no zone motion will occur. A larger current will result in a steady transfer of Y across the zone from the B/ liquid to the A/ liquid interface up the concentration gradient and the B phase will grow at the expense of the A. Similarly if a temperature gradient is applied across the zone such that the temperatures at the A/liquid and B/liquid interfaces are respectively T and T then both interfaces are in equilibrium at the concentration C. In this case no concentration gradient exists and the zone remains stationary. The application of a steeper temperature gradient set up a concentration gradient in the opposite sense (C C and the zone moves in the opposite direction i.e. B is grown at the expense of A.

The direction of motion of the zone is thus determined by the sign of the difference of the opposing thermodynamic forces.

We have appreciated that in the second embodiment where the unstable phase is made to grow at the expense of the stable phase there is in the melt no gradient of constitutional supercooling with respect to the freezing interface. The absence of a gradient of constitutional supercooling is a very important factor contributing to the homogeneity and perfection of crystals grown by any process.

In order to control the pressure at which the process takes place the bar 1 may be in some pressure-controlled enclosure, for example, a pressure bomb.

The orientation of the surface seed crystal A in contact with the liquid alloy may have a significant influence on the microscopic perfection of single crystal of certain materials. For example for certain diamond cubic semiconductors it may be desirable to make the surface orientation the (111) plane under certain conditions.

By way of example, a procedure for the growth of alpha (grey) tin from beta (white) tin will be described. Consider the arrangement shown diagrammatically in FIG. 4, in which a single crystal seed 11 of alpha tin is mounted on a refrigerated block 13. A block (or cylinder) 15 of beta tin is placed on top of the seed 11 but is separated from it by a thin film 17 of liquid mercury saturated with tin. Surface tension forces prevent the block 15 from touching the seed 11 directly. The thin liquid film 17 must wet both the surface of the seed 11 and that of the block 15 completely. An alternative arrangement can be used in which the block .15 and the seed 11 are physically separated by a thin mica annulus. lt is desirable to arrange that the surface of the seed 11 in contact with the liquid is a plane of the type {111} and that growth is made to proceed in the 111 direction. The growth of alpha tin occurs when beta tin dissolves in the mercury rich liquid film 17, diffuses across the film and finally crystallises on the surface of the seed crystal 11.

The growth of the alpha tin may be achieved by maintaining the whole system at the temperature of the refrigerated block 13 (for example by putting it in a refrigerator). This temperature must be 1&8 than 8 C. and its optimum value lies in the region of 20 C.

Alternatively a steady temperature gradient may be applied axially to the system by heating the block 15 from from above with a small heater (not shown). The optimum average temperature of the liquid film may not be the temperature employed in the first case and it must be determined by experiment, but it will lie in the range 20 C. to +20 C.

The whole apparatus is surrounded by a suitable inert ambient gas, or by a vacuum.

We claim:

1. In a process for growing single crystals of a first phase from a second phase of the same material in a system wherein one phase is metastable and the other phase is stable and the first phase is separated from the second phase by a thin liquid alloy zone, the improvement comprising imposing an external thermodynamic driving force on the said system, and directing the said driving force so as to cause the growth of the metastable phase at the expense of the stable phase.

2. A system as in claim 1 wherein the externally imposed thermodynamic driving force is a direct electric current.

3. A system as in claim 1 wherein the externally imposed thermodynamic driving force is a temperature gradient.

4. A system as in claim 1 in which gray tin is grown at the expense of white tin.

References Cited UNITED STATES PATENTS 2,813,048 11/1957 Pfann 148-1 2,932,562 4/1960 Pfann 10 3,378,409 4/1968 Hurle et a1. l481.6

L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner US. Cl. X.R. 23-301 

